This invention relates to an optical coordinate input device mounted in front of a display device for detecting a coordinate position on a display surface of the display device to input the same to a computer.
As a coordinate input device for manual input to a computer, there are various types such as, for example, an electromagnetic induction type, an electrostatic capacity type, a transparent electrode type and an optical detection type. Among those types, attention is directed to the optical detection type due to its relatively high reliability and operability. A coordinate input device of the optical detection type is normally mounted in front of a display device, and as a point on a display surface of the display device is touched by a finger to interrupt paths of light beams, a coordinate position of the point is specified without contacting a detecting device.
Such optical coordinate input devices are disclosed in U.S. Pat. Nos. 3,764,813, 3,775,560, 3,860,754 and 4,205,304, and optical coordinate input devices which include means for preventing an error of detection are disclosed in U.S. Pat. Nos. 4,243,879, 4,585,940 and 4,591,710.
Such an optical coordinate input device is normally located in front of an image display device such as a CRT display or an LCD and is used for coordinate input to a computer. An optical coordinate input device conventionally includes two light emitting element arrays each including a large number of light emitting elements and extending along two sides in directions of the X- and Y-axes of an outer periphery of a front surface of a screen of a CRT display or the like, and two light receiving element arrays each including a large number of light receiving elements and extending along the other two sides in the directions of the X- and Y-axes of the outer periphery of the front surface of the screen and in an opposing relationship to the light emitting elements. The light emitting elements are scanned to be successively selectively driven by a multiplexer while the opposing light receiving elements are successively selected so that when a light signal developed from one of the light emitting element is interrupted by a finger or the like during such scanning, the opposing light receiving element may detect it in order to produce coordinate signals.
An exemplary one of such conventional coordinate input devices of the optical detection type is shown in FIGS. 9 to 11. FIG. 9 is a perspective view of a coordinate input device mounted in front of a display device, FIG. 10 a partial cross sectional view showing a portion of the coordinate input device of FIG. 9 in which a light receiving element is located, and FIG. 11 a rear elevational view illustrating an internal structure of the coordinate input device of FIG. 9 with a rear cover removed.
Referring to FIGS. 9 to 11, the coordinate input device includes as principal components a frame member 1 of a substantially rectangular shape having an opening 2 at a central portion thereof, two rows of a large number of light emitting elements 4 such as LED's and two rows of a large number of light receiving elements 5 such as phototransistors arranged on a rear face of the frame member 1, that is, between the frame member 1 and peripheral edge portions of a display surface 3a of a display device 3 which may be a CRT (cathode ray tube) or the like, and an operating device 6 for detecting a position at which light paths 10 are interrupted within an operating area A in front of the display surface 3a of the display device 3 from combinations of the light emitting elements 4 and the light receiving elements 5 located in an opposing relationship to each other over the operating area A of the display device 3. Thus, an optical element train 13 in the form of a framework is formed by two rows 14 of the light emitting elements 4 and two rows 15 of the light receiving elements 5 which make opposing sides to each other.
The light emitting elements 4 and the light receiving elements 5 are fixedly mounted on a base plate 7 located within the frame member 1 such that their light emitting portions 4a and light receiving portions 5a may oppose to each other in individual pairs, and a light shield plate 8 is located in front of the light receiving portions 5a of the light receiving elements 5 and has a large number of light passing holes 8a of a predetermined area formed therein for allowing the light receiving elements 5 to receive only incoming light beams emitted from the individual paired light emitting elements 4. Further, in front of the light shield plate 8 and the light emitting elements 4, that is, on peripheral ridges of the operating area A, an infrared ray filter 9 for selectively passing infrared rays therethrough is mounted on the frame member 1 along the entire periphery of the operating area A. Accordingly, the light paths 10 which cannot be observed is formed within the operating area A in front of the display face 3a via the infrared ray filter 9.
Now, an inputting principle of the coordinate input device having such a construction as described above will be described. If, for example, a finger 11 touches a particular position on the display surface 3a for inputting as shown in FIG. 9, particular light paths 10 passing the position are interrupted by the finger 11, which will allow coordinates of the position to be specified. In particular, if the light emitting elements 4 of the light emitting element rows 14 are caused to successively emit light to effect scanning, light paths 10 interrupted by the finger 11 can be detected by the light receiving elements in the x and y directions. The light paths 10 are then determined by the operating device 6, and consequently the coordinates of the position are input to a host computer not shown.
However, in such a conventional coordinate input device as described above, since each of the light passing holes 8a in the light shield plate 8 located in front of the light receiving portions 5a of the light receiving elements 5 is required to have an area of such a degree that it will not degrade the detecting efficiency of the light receiving element 5, there is the possibility that an external light beam 16 of a high intensity may be introduced to any of the light receiving elements 5 obliquely from above as shown in FIG. 10 so that output of the light receiving elements 5 may be brought into a saturated condition. If the light receiving element 5 is brought into a saturated condition, it can no more detect light from the opposing light emitting element 4. In particular, if disturbant light of a high intensity is received by a light receiving element so that the impedance of the light receiving element is raised to a saturation level or an almost saturation level (as indicated at a in FIG. 8), when the light receiving element receives a light signal from an opposing light emitting element so that the impedance thereof is lowered, the saturation level will be reached in a moment (as indicated at b in FIG. 8). In this instance, the extent or degree of change of the impedance of the light receiving element is so small that reception of the light signal cannot be detected correctly, and accordingly a coordinate signal in error may readily be obtained.
Accordingly, if external light of a high intensity is introduced to the light receiving elements 5, they may operate in error, which will result in considerable deterioration in reliability of the entire device. Further, there is the possibility that if external light of a particularly high intensity such as flash light of a camera or sunlight should act on the light receiving elements 5, they may be oversaturated and thus resulted in destruction of the light receiving elements 5.